This invention relates to a method of adjusting a bridge circuit of a semiconductor pressure sensor under the wafer process and more specifically to a method of adjusting a bridge circuit of a semiconductor pressure sensor typical of a semiconductor pressure sensor fitted to the tip of a catheter for medical use.
When a mechanical stress is applied to a semiconductor crystal of silicon or the like, its resistance changes greatly because of the piezoelectric resistance effect and the perception of this fact has led to the development of a semiconductor pressure sensor. The process of making such a semiconductor pressure sensor comprises the steps of forming a strain gauge resistor by diffusion of impurity ions on the surface layer of a silicon single crystal, assembling four of the strain gauge resistors into a Wheatstone bridge, forming a recess in the back surface of the silicon monocrystal by etching, and disposing electrodes in suitable places on the surface thereof with the thin portion as a disphragm. When pressure is applied to the semiconductor pressure sensor, the diaphragm is deformed and the resistance value of the strain gauge resistor changes to a greater extent because of the piezoelectric resistance effect, so that a bridge output proportional to the pressure is obtainable.
The aforesaid semiconductor pressure sensor is extremely small in size and, particularly in the case of a semiconductor pressure sensor for medical use, a plurality of semiconductor pressure sensors are fitted to the tip of a catheter and inserted into a body. Accordingly, even in a semiconductor pressure sensor incorporating peripheral circuits such as a temperature compensating circuit, a pressure sensitivity compensating circuit and the like, a side of a chip should be about 1 mm or smaller in length.
Variations in the resistance value of each of the strain gauge resistors and diffused resistors at their formation and uninformity in the thickness of the diaphragm produced by the etching process result in variations in the characteristics of the semiconductor pressure sensor.
However, the semiconductor pressure sensor is extremely small in size and it is very difficult to measure the electrical characteristics while pressure is being actually applied and adjust the resistance balance of the bridge circuit. Actually, the measurement of such electrical characteristics while the pressure is being applied is omitted, so that only the electrical characteristics of the bridge circuit are measured to identify and discard what exceed a predetermined tolerance as inferior goods.
Accordingly, yields of the semiconductor pressure sensor are poor and, in addition to this problem, the measurement of electrical characteristics has decreased reliability because the degree of deformation of the diaphragm resulting from the pressure actually applied thereto is not measured.